In the semiconductor chip package industry, a chip carrying an integrated circuit is commonly mounted on a package carrier such as a substrate, a circuit board or a leadframe that provides electrical connections from the chip to the exterior of the package. In such packaging arrangement called flip chip mounting, where the active side of the chip is mounted in an upside-down fashion over the substrate, the chip and the substrate are usually formed of different materials having mismatched coefficients of thermal expansion. As a result, the chip and the substrate experience significantly different dimension changes when heated, and the mismatch in dimension changes causes significant thermally-induced stresses and warpage in the electrical connections between the chip and the substrate. If uncompensated, the disparity in thermal expansion can result in degradation in the performance of the chip, damage to the solder connections between the chip and the substrate, or package failure.
To reduce warpage and improve the reliability of flip chip packages, a number of approaches have been offered by the microelectronics industry. An encapsulant material or underfill is commonly used to fill the gap between the chip and the substrate to reduce the stress on the package during thermal cycling. Additionally, stiffeners are typically employed around the chip in the package assembly. The stiffeners are attached on the substrate and surround the chip to constrain the substrate in order to prevent chip warpage or other movement relative to the chip during thermal cycling. To further reduce the chance of warpage and promote thermal cooling of flip chip packages, heat spreaders are often mounted on top of the package to dissipate heat and counter-balance the forces exerted by the thermal expansion mismatches between at least the chip and the substrate.
Although heat spreaders and stiffeners reduce warpage, as the package is constrained by the heat spreader, there may be high stress on the solder joints between the chip and the substrate. Moreover, as the stiffener is attached onto the substrate, stress may be imposed on the substrate as it is being constrained by the stiffener. The stress on the substrate and chip may lead to chip performance degradation or package failure. Furthermore, for thermal applications the conventional heat spreader having a highly homogeneous conductive material is likely not suitable for component or system-level package designs, where non-uniform heating of the chip often occurs during operation.